U.S. patent application number 12/791336 was filed with the patent office on 2010-12-02 for transparent conductive film and electronic device including same.
Invention is credited to Motoyuki Hirooka, Makoto OKAI.
Application Number | 20100304131 12/791336 |
Document ID | / |
Family ID | 43220569 |
Filed Date | 2010-12-02 |
United States Patent
Application |
20100304131 |
Kind Code |
A1 |
OKAI; Makoto ; et
al. |
December 2, 2010 |
TRANSPARENT CONDUCTIVE FILM AND ELECTRONIC DEVICE INCLUDING
SAME
Abstract
The transparent conductive film according to the present
invention comprises graphene platelets which overlap one another to
form a multilayer structure. The average size of the graphene
platelets is 50 nm or more and the number of layers of the graphene
platelets is 9 or less. The transparent conductive film has an
electrical resistivity of 1.0.times.10.sup.-6 (.OMEGA.m) or less
and a light transmission at a wavelength of 550 nm of 80% or
more.
Inventors: |
OKAI; Makoto; (Tokorozawa,
JP) ; Hirooka; Motoyuki; (Kumagaya, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
43220569 |
Appl. No.: |
12/791336 |
Filed: |
June 1, 2010 |
Current U.S.
Class: |
428/333 ;
423/445R; 428/332; 428/408; 977/832; 977/834 |
Current CPC
Class: |
C01B 2204/32 20130101;
H01L 51/442 20130101; Y10T 428/30 20150115; Y10T 428/26 20150115;
Y02E 10/549 20130101; Y10T 428/261 20150115; C01B 2204/04 20130101;
B82Y 30/00 20130101; C01B 32/186 20170801; H01B 1/04 20130101; B82Y
40/00 20130101; C01B 2204/02 20130101; C01B 2204/22 20130101 |
Class at
Publication: |
428/333 ;
423/445.R; 428/408; 428/332; 977/832; 977/834 |
International
Class: |
H01B 1/04 20060101
H01B001/04; C01B 31/00 20060101 C01B031/00; B32B 9/00 20060101
B32B009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2009 |
JP |
2009-132656 |
Claims
1. A transparent conductive film, comprising graphene platelets
overlapping one another to form a multilayer structure.
2. The transparent conductive film according to claim 1, wherein
average size of the graphene platelets is 50 nm or more, and number
of layers of the multilayer structure is 9 or less.
3. The transparent conductive film according to claim 1, wherein:
average size of the graphene platelets is 50 nm or more; each of
the graphene platelets consists of single atomic layer or a
plurality of atomic layers less than 9; and total number of the
atomic layers in the multilayer structure is 9 or less.
4. The transparent conductive film according to claim 1, wherein
the transparent conductive film has an electrical resistivity of
1.0.times.10.sup.-6 (.OMEGA.m) or less and a light transmission at
a wavelength of 550 nm of 80% or more.
5. A transparent conductive film-on-substrate, comprising a
substrate and the transparent conductive film according to claim 1
formed on the substrate, the substrate made of a glass or a
plastic.
6. The transparent conductive film-on-substrate according to claim
5, wherein the transparent conductive film is formed on the
substrate by a chemical vapor deposition method.
7. The transparent conductive film-on-substrate according to claim
5, wherein the transparent conductive film is formed by applying at
least one material selected from polyvinyl alcohol, polyvinyl
chloride, polyvinyl pyrrolidone, polyacrylamide, polyethylene
terephthalate, and hydroxypropyl cellulose on the substrate and
then by heat-treating the substrate.
8. An electronic device comprising the transparent conductive film
according to claim 1.
9. The electronic device according to claim 8, wherein the
electronic device is a flat panel display device, a touch panel, or
a photovoltaic cell.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese patent
application serial no. 2009-132656 filed on Jun. 2, 2009, the
content of which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to transparent conductive
films formed of graphene and electronic devices such as flat panel
display devices including such transparent conductive films.
[0004] 2. Description of Related Art
[0005] Conventionally, ITO (Indium Tin Oxide) has been widely used
as a material for transparent conductive films (see, e.g., JP-A Hei
6 (1994)-172995). ITO is an inorganic compound of tin oxide and
indium oxide, typically 5-10 mass % of tin oxide by mass. Since the
indium (In) is a rare metal, its availability is easily affected by
a market value and there is concern over its future availability.
Therefore, it is desirable to develop alternative transparent
conductive films.
SUMMARY OF THE INVENTION
[0006] In recent years, graphenes (also called graphene sheets) are
come into the spotlight as attractive electronic materials. Herein,
graphenes are a sheet of six-membered carbon rings which does not
form a closed surface, and are formed by connecting numerous
benzene rings two-dimensionally. Carbon nanotubes are formed by
rolling up a graphene sheet into a tubular structure. Graphites are
formed by stacking multiple graphene sheets. Each carbon atom in a
graphene sheet has an sp.sup.2 hybrid orbital, and delocalized
electrons are present at opposite surfaces of a graphene sheet.
[0007] The following typical physical properties of graphenes have
been reported: (a) The carrier mobility is in the order of 200,000
cm.sup.2/Vs, which is one order of magnitude higher than those of
silicon (Si) crystals and is also higher than those of metals and
carbon nanotubes. (b) The 1/f noises of typical nanodevices can be
significantly reduced. (c) The refractive index is negative. (d)
The surface electrons behave as if they have no mass. Because of
these properties, graphenes are identified as a candidate for
post-silicon electronic materials.
[0008] Under the above circumstances, it is an objective of the
present invention to provide transparent conductive films formed of
inexpensive carbon materials, instead of rare metals such as
indium.
[0009] (I) According to one aspect of the present invention, there
is provided a transparent conductive film having a multilayer
structure in which graphene platelets overlap one another.
[0010] In the above aspect (I) of the invention, the following
modifications and changes can be made.
[0011] (i) Average size of the graphene platelets is 50 nm or more,
and number of layers of the multilayer structure is 9 or less.
[0012] (ii) Each of the graphene platelets consists of single
atomic layer or a plurality of atomic layers less than 9, and total
number of the atomic layers in the multilayer structure is 9 or
less.
[0013] (iii) The transparent conductive film has an electrical
resistivity of 1.times.10.sup.-6 .mu.m or less and a light
transmission at a wavelength of 550 nm of 80% or more.
[0014] (II) According to another aspect of the present invention,
there is provided a transparent conductive film-on-substrate
comprising a substrate and the above-mentioned transparent
conductive film formed on the substrate, the substrate made of a
glass or a plastic.
[0015] In the above aspect (II) of the invention, the following
modifications and changes can be made.
[0016] (iv) The transparent conductive film is formed on the
substrate by a chemical vapor deposition method.
[0017] (v) The transparent conductive film is formed by applying at
least one material selected from polyvinyl alcohol, polyvinyl
chloride, polyvinyl pyrrolidone, polyacrylamide, polyethylene
terephthalate, and hydroxypropyl cellulose on the substrate and
then by heat-treating the substrate.
[0018] (III) According to still another aspect of the present
invention, there is provided an electronic device comprising the
abovementioned transparent conductive film.
[0019] In the above aspect (III) of the invention, the following
modifications and changes can be made.
[0020] (vi) The electronic device is a flat panel display device, a
touch panel, or a photovoltaic cell.
Advantages of the Invention
[0021] According to the present invention, it is possible to
provide transparent conductive films formed of only inexpensive
carbon materials, instead of rare metals such as indium, and to
provide electronic devices including such transparent conductive
films.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is an explanatory model illustration showing a
perspective view of a transparent conductive film in accordance
with the present invention.
[0023] FIG. 2 is an explanatory graph showing the correlation
between the size of graphene platelet and the electrical
resistivity in accordance with the present invention.
[0024] FIG. 3 is a schematic illustration showing a plan view of a
transparent conductive film in accordance with a first embodiment
of the present invention.
[0025] FIG. 4 is a schematic illustration showing a cross-sectional
view of a liquid crystal display device including a transparent
conductive film in accordance with a second embodiment of the
present invention.
[0026] FIG. 5 is a schematic illustration showing a cross-sectional
view of a touch panel device including a transparent conductive
film in accordance with a third embodiment of the present
invention.
[0027] FIG. 6 is a schematic illustration showing a cross-sectional
view of a photovoltaic cell device including a transparent
conductive film in accordance with a fourth embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] First, a basic idea of the present invention will be
explained below with reference to FIGS. 1 and 2.
[0029] FIG. 1 is an explanatory model illustration showing a
perspective view of a transparent conductive film in accordance
with the present invention in order to calculate the electrical
resistivity of the film. Assuming that graphene platelets, each of
which measuring L (unit: .mu.m) per side, overlap one another by
half its length to form a conducting channel from left to right in
the figure. In FIG. 1, the model comprises three such channels
formed in parallel. Based on this structure, the electrical
resistivity (unit: .OMEGA.m) is calculated in the case of forming a
1-.mu.m-square wiring. The contact resistance R.sub.gg between
L-.mu.m-square graphene platelets is given by the following
equation (Eq.) 1.
R gg = r cgg L .times. L 2 = 2 r cgg L 2 Eq . 1 ##EQU00001##
[0030] where r.sub.cgg is the interface resistance between graphite
bulks.
[0031] Since each 2/L of this R.sub.gg are connected in series to
form one channel, 1/L of which is in parallel, the electrical
resistance of 1-.mu.m-square graphene platelets Rt is given by the
following Eq. 2.
R t = R gg 2 L L = 4 r gg L 2 Eq . 2 ##EQU00002##
[0032] Furthermore, the electrical resistivity of the wiring is
given by multiplying Rt by the cross-sectional area of the wiring
and dividing by the length of the wiring. Herein, the electrical
resistance of graphene itself can be ignored since it is negligibly
smaller than the contact resistance therebetween, and r.sub.cgg of
10 .OMEGA..mu.m.sup.2 is assumed. Also, the thickness of a graphene
platelet is assumed as 0.34 nm, which means a graphene platelet
comprises a single-atom graphene layer (single graphene sheet).
[0033] The equations described above apply to the case of graphene
platelets in a 2-layer stack. In the case of graphene platelets in
a 9-layer stack, since 8 layers of channels are connected in
parallel along the longitudinal direction, Rt is given by dividing
Eq. 2 by 8. FIG. 2 is an explanatory graph showing the correlation
between the size of graphene platelet and the electrical
resistivity in accordance with the present invention, as the
calculation results. As shown in FIG. 2, it was revealed that the
electrical resistivity of the wiring decreased with increasing the
size of graphene platelet.
[0034] Meanwhile, the light absorption of a graphene sheet at a
wavelength of 550 nm is about 2.3%. Therefore, in the case of
graphene platelets in a 2-layer stack (two graphene sheets), the
light transmission is about 95.5%, and in the case of graphene
platelets in a 9-layer stack (nine graphene sheets), about 81.1%.
Since the light transmission of a commercially available ITO
(Indium Tin Oxide) film is typically 80% and the electrical
resistivity is 1.5.times.10.sup.-6 (.OMEGA.m), graphene platelets
that are 50 nm or larger in size can achieve performance comparable
to that of ITO. In addition, since the light transmission of a
commercially available ITO film is 80%, a wiring film formed of
graphene platelets in a 9- or less-layer stack can achieve
performance comparable to that of an ITO film.
[0035] As described above, 50-nm or larger-square graphene
platelets in 9- or less-layer stack can achieve a light
transmission of 80% or more, and an electrical resistivity of
1.0.times.10.sup.-6 (.OMEGA.m) or less. Such performances are equal
to, or higher than, those of a conventional ITO film. In other
words, there can be provided a graphene transparent conductive film
(also called transparent electrode) that can replace an ITO film.
Moreover, a graphene platelet comprising a plurality of atomic
layers is regarded as the same as a single-atom graphene layer, if
the total number of atomic layers of graphene platelets in the
multilayer structure is 9 or less.
[0036] Preferred embodiments of the present invention will be
described below. However, the invention is not limited to the
specific embodiments described below, but various combinations of
its features are possible within the scope
First Embodiment of Present Invention
[0037] A first embodiment of the present invention will be
described with reference to FIG. 3. FIG. 3 is a schematic
illustration showing a plan view of a transparent conductive film
in accordance with a first embodiment of the present invention. As
shown in FIG. 3, graphene platelets 301 overlap one another to form
a multilayer structure, from which a transparent film with a low
electrical resistivity can be formed. Since actual graphene
platelets come in various shapes, the size of graphene platelets is
defined as an average of the arithmetic mean between the longest
diameter and the shortest one of each platelet. The average size of
graphene platelets can be measured either by observing a thin film
of graphene separated from a substrate under a transmission
electron microscope, or by observing a thin film of graphene formed
on a substrate under a scanning tunneling microscope.
[0038] As shown in FIG. 2, the electrical resistivity of a graphene
transparent conductive film depends on the average size of graphene
platelets, and the electrical resistivity decreases with increasing
the average size. Furthermore, the light transmission depends on
the number of layers of graphene platelets (the total number of
atomic layers in the multilayer structure), and the light
transmission increases with decreasing the number of layers. For a
graphene transparent conductive film to achieve the conventional
properties of ITO, an electrical resistivity of 1.0.times.10.sup.-6
(.OMEGA.m) or less, and a light transmission at a wavelength of 550
nm of 80% or more, it needs to be formed of graphene platelets that
are 50 nm or larger in average size and are in a 9- or less-layer
stack.
[0039] Such a graphene transparent conductive film can be formed on
the surface of a glass substrate by a chemical vapor deposition
(CVD) method using acetylene gas as a starting material. In this
method, a glass substrate is placed in a reaction tube of a growth
furnace to be heated to 550.degree. C. and acetylene gas is
introduced to form a graphene transparent conductive film on the
entire surface of the glass substrate. That is, each graphene
platelet is grown parallel to the substrate surface. The number of
graphene layers (total number of atomic layers) can be controlled
by adjusting the flow of acetylene gas and the growth time. For
example, a graphene transparent conductive film formed of 7-layer
graphene (seven atomic layers) could be formed by setting the
acetylene gas flow at 0.5 sccm (standard cc/min) and the growth
time for 10 minutes. It was demonstrated that any hydrocarbon gas
was able to be used, instead of acetylene, as a starting material
for the chemical vapor deposition, and that a graphene transparent
conductive film could be formed at growth temperatures of
400.degree. C. or higher.
[0040] Other than by the above-mentioned chemical vapor deposition
method, a similar graphene transparent conductive film was able to
be formed by applying a solution of polyvinyl alcohol and volatile
solvent onto a glass substrate by spinner coating, and by
heat-treating the substrate at 500.degree. C. under an atmosphere
of nitrogen or other inert gas. On the condition that the heat
treatment temperature was 400.degree. C. or higher, the closer the
heat temperature was to the upper temperature limit of the
substrate, the lower the electrical resistivity of the formed
graphene transparent conductive film was. It was demonstrated that
polyvinyl chloride, polyvinyl pyrrolidone, polyacrylamide,
polyethylene terephthalate, or hydroxypropyl cellulose could also
be used instead of polyvinyl alcohol as a coating material. The
coating material can also be applied by dipping, printing, or any
method other than spinner coating. Furthermore, any substrate, such
as a plastic substrate, with a heat resistance of 400.degree. C. or
higher can be used as the substrate.
Second Embodiment of Present Invention
[0041] A second embodiment of the present invention will be
described with reference to FIG. 4. FIG. 4 is a schematic
illustration showing a cross-sectional view of a liquid crystal
display device including a transparent conductive film in
accordance with a second embodiment of the present invention. As
shown in FIG. 4, in the liquid crystal display device, liquid
crystal 403 is sandwiched between a lower glass substrate 401
provided with a lower transparent electrode 402, which is wired in
the horizontal direction in the plane, and an upper glass substrate
406 provided with an upper transparent electrode 404, which is
wired in the vertical direction in the plane, and a color filter
405. Light is irradiated from the backside of the lower glass
substrate 401 by a backlight 407 to create an image on the side of
the upper glass substrate 406.
[0042] The graphene transparent conductive films of the present
invention were used for the lower transparent electrode 402 and the
upper transparent electrode 404 of this liquid crystal display
device. These graphene transparent conductive films had the
electrical resistivity of 1.0.times.10.sup.-6 (.OMEGA.m) and the
light transmission at a wavelength of 550 nm of 80%. In addition to
liquid crystal display devices, the graphene transparent conductive
films of the present invention can be used for other display
devices including any flat panel display such as an organic light
emitting display and an inorganic light emitting display.
Third Embodiment of Present Invention
[0043] A third embodiment of the present invention will be
described with reference to FIG. 5. FIG. 5 is a schematic
illustration showing a cross-sectional view of a touch panel device
including a transparent conductive film in accordance with a third
embodiment of the present invention. As shown in FIG. 5, in the
touch panel, a lower substrate 501 provided with a lower
transparent electrode 502, which is wired in the horizontal
direction in the plane, and an upper substrate 503 provided with an
upper transparent electrode 504, which is wired in the vertical
direction in the plane, face each other at a certain interval with
a spacer, etc. A touch on the upper substrate 503 causes the upper
transparent electrode 504 and the lower transparent electrode 502
to come into contact at the touched position, thereby detecting the
location of the touched position.
[0044] The graphene transparent conductive films of the present
invention were used for the lower transparent electrode 502 and the
upper transparent electrode 504 of this touch panel. These graphene
transparent conductive films had the electrical resistivity of
1.0.times.10.sup.-6 (.OMEGA.m) and the light transmission at a
wavelength of 550 nm of 80%.
Fourth Embodiment of Present Invention
[0045] A fourth embodiment of the present invention will be
described with reference to FIG. 6. FIG. 6 is a schematic
illustration showing a cross-sectional view of a photovoltaic cell
device including a transparent conductive film in accordance with a
fourth embodiment of the present invention. As shown in FIG. 6, in
the photovoltaic cell, a light absorbing layer 603 to generate
electricity by absorbing light is sandwiched between a lower
substrate 601 provided with a lower electrode 602 and an upper
substrate 605 provided with an upper transparent electrode 604.
[0046] The graphene transparent conductive film of the present
invention was used for the upper transparent electrode 604 of this
photovoltaic cell. The graphene transparent conductive films had
the electrical resistivity of 1.0.times.10.sup.-6 (.OMEGA.m) and
the light transmission at a wavelength of 550 nm of 80%.
[0047] Although the invention has been described with respect to
the specific embodiments for complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
* * * * *